The role of interleukin 2 in inducing Ig production in a pokeweed mitogen-stimulated mononuclear cell system

Cyclosporin A (CsA) has been found previously to block mitogen-stimulated T cell proliferation and production of discrete T cell-derived lymphokines such as interleukin 2 (IL 2) and interferon (IFN)-γ. In addition, CsA blocks pokeweed mitogen (PWM)-driven T cell-dependent differentiation of B cells into immunoglobulin (Ig)-secreting cells. Recently, we reported that CsA (1 μg/ml) inhibited PWM-induced T cell production of IL 2 and IFN-γ, but supernatants retained B cell differentiation factor (BCDF)-like activity. The present study demonstrates the ability of CsA to suppress T cell functions in PWM-driven Ig production in mononuclear cells (MNC), and the capacity of exogenous T cell lymphokines to reverse CsA-induced suppression. CsA profoundly suppressed PWM-driven PFC formation (>95%). However, Ig production was substantially reconstituted by the addition of IL 2 at concentrations of 10 to 50 U/ml. In contrast, no effects were observed by the addition of IFN-γ or BCGF. The kinetics of CsA inhibition of Ig production and IL 2 secretion was found to be closely related. In addition, to obtain effective reconstitution in the CsA-treated PWM-MNC system it was necessary to add IL 2 at the initiation of culture. T cells themselves were also required for B cell differentiation in this system. However, surface Ig⁺ cells obtained by cell sorting after 3 days of culture could differentiate in the absence of T cells but only in response to IL 2, not in response to IFN-γ or BCDF. Thus, in PWM-driven B cell differentiation T cells are necessary early in culture, whereas IL 2 is essential from the initial stage of B cell activation through the final stage of B cell differentiation.